[Rcpp-devel] Passing an exposed function to another C++ function as a parameter

[Manuel Castejón Limas]

Dear all,

I'm rewriting the AMORE package using Rcpp --- in fact it's more like I'm
having a lot of fun while rewriting the AMORE package thanks to Rcpp.
Nevertheless, I'm facing this little problem that I hope it would be pretty
easy for you to solve.


Let's consider the C++ function Tanh_f0

 double

Tanh_f0(double inducedLocalField)

  {

    return tanh(inducedLocalField);

  }


After compilation using inline, Tanh_f0 is exposed to R using Module and
accessible through actMod$Tanh_f0


actMod$Tanh_f0(0.1)

# [1] 0.09966799


Now, I want to pass actMod$Tanh_f0 as a parameter of a function in order to
use the original C++ function Tanh_f0.



testCode <- '  Rcpp::Function fx (myfun);

double result = as<double>(fx(x));

return wrap( result );

    '

usingTanhf0 <- cfunction(sig=signature(myfun="function", x="numeric"),
body=testCode,…)


And that indeed works, but when compared to using a simple tanh function it
shows quite a bad performance.



testCode <- '  double value = as<double>(x);

double result = tanh(value);

return wrap( result );

    '


usingTanh <- cfunction(sig=signature(x="numeric"), body=testCode, … )

 benchmark(usingTanhf0(myfun=actMod$Tanh_f0, x=0.1) , usingTanh( x=0.1),
columns=c("test", "replications", "elapsed", "relative"), order="relative",
replications=1000)

#                                           test replications elapsed
relative

# 2                           usingTanh(x = 0.1)         1000   0.004
1

# 1 usingTanhf0(myfun = actMod$Tanh_f0, x = 0.1)         1000   0.080
20


Looks like having to go the R way to get access to Tanh_f0 has a high cost.
In order to be faster, it would be great if I could have access to the
original Tanh_f0 which I guess is pointed to by  actMod$Tanh_f0, may be at
the address 0x100153d30 ?


actMod$Tanh_f0

# internal C++ function <0x100153d30>

#     signature : double Tanh_f0(double)

After having had a look at RppDE sources, I guess the solution would be to
pass the function as an external pointer; something like :

testCode <- '

typedef double (*funPtr)(double) ;

Rcpp::XPtr<  funPtr > fx (myfun);

double result = (*fx)(0.1);

return wrap( result );

'

testCodefun <- cfunction(sig=signature(myfun="C++Function"),
body=testCode,…)

result <- testCodefun(myfun=actMod$Tanh_f0)

But this does NOT work and crashes R.

Any hints?

Thank you for your patience in reading such long message!
Manuel
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